A filename consisting of nothing but ASCII Bell characters (0x07) is valid. Those are non-printable characters that (used to) make a sound from the PC's speaker

A filename consisting of nothing but ASCII Bell characters (0x07) is valid. Those are non-printable characters that (used to) make a sound from the PC's speaker. POSIX filenames can be sound, not text. I'd agree it'd be nice if we could restrict filenames to valid UTF-8. But that's not the API that existing filesystems provide, nor what (most of) the existing OSes enforce.